Method of molding insulating material



May 6, 1952 l.. c. AlcHER METHOD OF MOLDING INSULATING MATERIAL Filed July 27, 1946 A TORNEY Patentecl May 6, 1952 METHOD oF MOLDING INSULATING MATERIAL Louis C. Aicher, Wauwatosa, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation' of Delaware Application| July 27, 1946, Serial No'. 686,742

6 Claims. (Cl. 1M- 2.6)

This invention relates in general to improvements in methods of molding insulating material and more particularly to a method of forming articles of cellulosic material having a substantially uniform wall thickness.

The manufacture of electrical apparatus such as transformers and dynamoelectric machines requires the use of elements of insulating material which have a substantially uniform wall thickness and are adapted to be made by deformingraw material in sheet form. Where insulation is protected against moisture the raw material may be a laminated cellulosic material such as fullerboard. To make insulating elements in which the raw material is to be considerably deformed from the fiat sheet form the fullerboard, or other raw material, may first be soaked in a 'suitable vaporizable softening liquid such as water or alcohol to render it more pliable. The wet material is then pressed between heated dies to cause it toassume the desiredr shape and tov expel the Water or other liquid absorbed during the soaking operation. Heat transmission however takes place slowly through fullerboard and the dies therefore may only be heated to a relatively'lowy temperature to avoid overheating of the superficial portions of the article being made, so that the drying process requires an excessive length oftime. Y

If an, alternating electric iield of suitable intensity and frequency is established between the dies, the wet fullerboard vmay become yheatedthroughout to a relatively uniform temperature by the production ofdielectric losses therein, so that theheating rate may be v'accelerated to a considerable extent without locally overheating the material. The dies nevertheless should be brought to substantially the temperature of ebullition of the softeningliquid, which will be assumed hereinafter to be water, so as not to chill the supercial portions of the material. To eliminate the necessity of separate preheating means for the dies it is advantageous to preheat the dies by applying an alternating electric field to a previously molded article or to a body of other dry insulating material inserted between the dies.

. As the dies and the material contained therebetween constitute a variable capacitive load for the oscillator,'it is advantageous to connect an adjustable capacitor in series therewith to per- 2 onance. After the article being made reaches the temperature of ebullition of water the voltage impressed on the dies may be reduced to prevent production of steam under excessive pressure within the article. It is possible to mold several articles simultaneously in dies connected in parallel in the circuit because the production of heat within each article slows up in the course of its drying, so that the moisture content tends to reach the desired value simultaneously in the different articles.

It isV therefore an object of the present invention to Aprovide an improved method of molding an article of insulating material from a sheet of laminated cellulosic insulating material requiring molding'in the wet stage.

Another object of the present invention is to provide an improved method of molding an article of insulating material in which the article is held to the desired shape while moisture is being expelled therefrom.

Another object of the present invention is to provide an improved method of simultaneously molding a plurality of articles of insulating material from wet stock in which the drying of the different articles tends to proceed at a uniform rate.

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the accompanying drawing in which:

Fig. 1 diagrammatically illustrates a system comprising two hydraulic presses and source of alternating current for simultaneously molding two articles of insulating material from wet sheet stock by dielectric heating;

. inated cellulosic insulating material such as fullerboard. Dies 6, 1 may have any suitable size and mit tuning the circuit of the dies to resonance for the frequency of the voltage of the oscillator. If necessary, an inductive conductor may be connected across the dies and the capacitor to assist shape depending on the size and shape of the articles to be made, and it will be assumed that the dies are adapted to mold a piece of curved insulating channel 8. 'Ihe dies are made at least partly of a suitable conductive material, and the conductive portions thereof may either be in contact with the material to be molded or may be the capacitor in bringing the circuit into resembedded in the insulating material.` Satisfactory results have been consistently obtained with dies made of steel.

To reduce the cost of the dies required for molding sets of channels having the same curvature but different widths or thicknesses, die 6 may be built up of end plates 9 machined to provide the desired radius at the juncture of the web with the anges of the channel, and of spacer plates I of different thicknesses of which a suitable number may be stacked betweenV the end plates to match the desired outer prole of the channel. Die 1 may likewise be built up of end plates I2 and spacer plates I3 to match the desired inner profile ofA the channel. The portions of the dies which are not in contact with channel 8 are s'o shaped as to permit impression4 of la substantial voltage therebetween without hash-over and as to provide a path for the escape of steam from the channel. Ports I4 may also be provided in the dies to relieve excessive steam pressures which may build up within the channels.

Dies 6, 1 may be supported and relatively displaced by any suitable means such as a hydraulic press I5. One of the dies, die 6 for example, is mounted on press I5 through a block of suitable insulating material I6 such as Bakelite. The different component parts of die 1 may 'be mounted on a stand I1 resting in a tray I8 for collecting water expelled from channel 8 and discharging the collected water into adrain I9. The ram of press I5 may be actuated by means of suitable liquid supplied thereto from a reservoir 2| through a pump 22and a four-way valve 23. Pump 22 may be driven by any suitable means such as an electric motor 24. The pressure exerted on the ram on atleast the downstroke may preferably be ascertained by means of a pressure gauge 25. A gauge block 26 may be provided for ascertaining the spacing between the dies and, if desired, for serving as a stop to limitl the travel of die 6.

A voltage of suitable magnitude and frequency may be impressed between dies 6, 1 from any suitable known oscillator, generally designated 21. The major portion of the connections between the dies and the oscillator preferably consists of coaxial conductors 28, 29 of relatively low impedance. The portion of the load circuit of oscillator 21, consistingof dies 6, 1 and the connections between the dies and the -coaxial conductors, is preferably arranged to be resonant at the` frequency'impressed thereon and to radiate electromagnetic waves to the lowest possible extent. The resonant condition of the circuit may be established by means of a variable capacitor 3| connected between conductor v28 and die 6 or otherwise inserted vin `series in the circuit. A dial 32 associated with a'pointer mounted on the shaft of the movable plate of capacitor 3l permits setting the plate in any preselected position. If necessary, an inductive conductor 33 forming a loop, may be connected between conductors 28, 29 to produce a coarse tuning of the circuit, fine tuning then being effected by VV'adjustment oi capacitor 3|. The tuned condition of the circuit may be ascertained by means of an ammeter in the plate circuit of the oscillator or by ymeans of an Vammeter 34 connected in series with the dies.

Oscillator 21 may be supplied from any suitable current supply such as a circuit 35 energized from a suitable alternating current generator (not shown). The oscillator proper or vthe connections of circuit `35 therewith `may include yany suitable known voltage regulating means such as a transformer 36 provided with a tap changer 31. The connections between transformer 36 and circuit 35 may be interrupted by means of a switch 38. Operation of tap changer 31 and of switch 38 may be controlled manually or by means of a timer, generally designated 39. If the output of oscillator 21 is suihciently large, dies 6, 1 may be connected in parallel with one or more additional pairs of dies such as dies 4I, 42, to permit simultaneous molding of a plurality of articles.

In operation, assuming that an article is to be molded in dies 6, 1, the dies are preferably first heated to substantially the boiling point of water. Such heating is conveniently effected by inserting between the dies a previously molded dry channel or dry strips of fullerboard, closing the dies, and closing switch 38 to cause oscillator to impress an alternating voltage on the dies. If the ouput voltage of oscillator 21 has a value of the order of several thousand volts and a frequency of the order of two to thirty megacycles per second, the insulating material inserted between dies 6, 1 gradually heats up by production therein of dielectric losses. The heat generated in the material is transmitted to dies 6, 1 by conduction, and the operation is continued until the dies have reached the desired temperature.v While the dies are thus being heated or while a channel is already in process o'f being molded in the dies, a sheet of fullerboard of suitable size and thickness is placed to soak in 'a ltray of water. The sheet absorbs a considerable amount of water and increases in volume while becoming more pliable. rThe sheet may further vbe kneaded by hand to loosen the bond between the plies thereof and thereby facilitate subsequent deformation of the sheet within the dies.

Switch 38 is opened and the heated die's are separated, and the material previously confined therebetween is removed. The dies are then preferably coated withv a lm of liquid lubricating material insoluble in water to prevent sticking of the -fullerboard therein and to reduce corrosion of the dies. vSuch liquid lm may be 'obtained by spraying the dies with a liquid hydrocarbon or by rubbing the dies with a lcake of paraffin, which melts at a temperature well bebelow the boiling 'point of water.

The softened sheetvof fullerboard is then laid on die 1, preferably with the bers of the fuller- Aboard directed across Athe die to reduce the danger of tear-ing the sheet when the dies are closed. The -sheet is shaped by hand over die 1, the loosened structure -of the sheet enabling the plies thereof to yield and to slide relatively to each other so as to permit the sheet to be applied to die 1 -substantially without wrinkles.

Valve 23 is then actuated to cause the 'ram of press I5 to force die 6 on die 41. The sheet of fullerboard is thus compressed between the'dies and a large portion of the water Aabsorbed `by the sheet is thus expelled,l collected in tray I 8 and discharged to drain I9. The sheet, being conned between the dies, is thereby 'molded to the desired shape. Valve 23may be so actuated as to admit fluid from pump 22 to the upper cylinder of press I5 until gauge 25 registers a predetermined pressure which has previously been found adequate for compressing the web of channel 8 to the desired thickness.

The ram may also be caused to drop until die 6 abuts a gauge block or stop 246 resting on base I1. If block 25 is of insulating material the block may be left 1in .place during lthe dielectric heating operation. If block 26`is n'ade of metai'it must be removed before application of voltage to dies 6. 1. For the latter purpose it may be convenient to observe the pressure indicated by gauge 25 at -the moment of engagement of die 6 with block 26, release the pressure slightly by movement of valve 23 to permit die 6 to rise, remove block 26l and bring the pressure to the previously observed value to return die 6 to the position at which it engaged block 26.

Switch 38 may then be reclosed to cause oscillator 21 to impress an alternating voltage on the dies. Capacitor 3l is adjusted to the -value rendering the die circuit resonant, and tap changer 31 or other voltage regulating device is adjusted manually or by means of timer 39 to cause impression on the die of a relatively high voltage to raise the temperature of channel 8 to the boiling point of water at the highest possible rate. Whenichannel vily-remaches;the boiling point of water, the remainder of the water absorbed thereby duringvthe soaking voperation gradually escapes between the dies in the form of steam. The watercontent of channel 8 thus gradually decreases, whereby the capacity introduced `by channel 8in the'output circuit of oscillator, 21 gradually decreases. Capacitor 3i therefore should bereadjusted at frequent intervals during the `heating process to maintain the balance between the inductance and capacitance of the die circuit. I

As channel 8 is confined between dies 6. 1 over the greatest portion of its surface, the escape of steam therefrom can take place only over a relatively small area. The rate of vaporization of the water contained in channel 8 should therefore be regulated to prevent building up of excessive pressures within the channel. Such regulation may be effected by actuation of tap changer 31 either manually or by timer 39 in accordance with a previously determined schedule.

The heating operation may be terminated when the water absorbed by the fullerboard during the soaking operation has been substantially entirely expelled and the moisture content of channel 8 has returned to substantially the normal value for dry fullerboard, which is in the neighborhood of six per cent at room temperature. Channels of a uniform size require a substantially uniform heating time, and opening of switch 38 to terminate the heating operation may therefore be effected by timer 39. Switch 38 may also be opened when return of channel 8 to the dry state is evidenced by the particular adjustment of capacitor 3l required for bringing the circuit of the dies back to the tuned condition.

Valve 23 may then be actuated to cause the ram of press I5 to separate dies 6, 1 and channels may be removed from the dies. Another sheet of v'wet fullerboard may then be inserted in the dies for forming another channel in the manner above set forth. It will be understood however that the different steps of `the process may be performed either in the exact sequence indicated above or in any other order that may be found convenient or advantageous.

When two or more pairs of dies such as dies 6, 1 and 4I, 42 are connected in parallel, the dies are preferably heated simultaneously in the manner above set forth. The heated dies are then charged with wet sheets of fullerboard and reclosed. The different dies have the electrical properties of a plurality of parallel capacitors and the die circuit may again be tuned by adjustment of the variable capacitor common thereto. During the drying operation, the rate of heat production in each/channel gradually decreases to an extent determined by the decrease in thewater content of the channel. The channel which has the higher water content therefore heats faster than the channel having the lower water content; The heating rate of the different channels thus adjusts itself automatically in such manner that the dierent channels heat at substantially the same' rate and that the moisture content of the channels reaches the desired value substantially simultaneously-in different channels.

Although but one embodiment of the present invention has been illustrated and described, it is apparent to one skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims. `Fea-" tures disclosed but not claimed herein are claimed in a copending' application Serial No. 687,738 of Louis- C. Aicher and Fred J. Vogel, filed August 1.- 1946.

It is claimed and desired to secure by Letters Patent: f

l. A methodof molding an article of insulating material comprising the steps of closing a pair of conductive dies on a piece of dry insulating ma terial shaped to t between said dies, connecting said dies to a source of high frequency electric current to cause heating of said piece of insulating material and of said dies to substantially the temperature of ebullition of a softening liquid. soaking a sheet of laminated cellulosic insulating material in said softening liquid, disconnecting said diesv from said source, substituting said sheets for said piece, pressing said sheet between said dies, and reconnecting said dies with said source to vaporize the liquid absorbed by said sheet.

2. A method of molding an article of insulating material comprising the steps of closing a pair of conductive dies on a piece of insulating material shaped to fit between said dies, connecting said dies to a source of high frequency electric current to cause heating of said piece of insulating material by production of dielectric losses therein and conduction of heat from said piece of insulating material to said dies to raise said dies to substantially the temperature of ebullition of a softening liquid, disconnecting said dies from said source, separating said dies and removing saidV piece therefrom, rubbing said dies with a cake of parain to form a liquid lubricating film thereon, soaking a sheet of laminated cellulosic `insulating material in said softening liquid, knea'ding said sheet to loosen the bond between the 4plies thereof, shaping said sheet on one of saidY dies, pressing said sheet between said dies to expel a portion of the softening liquid absorbed by said sheet and mold said'sheet to the desired shape, and reconnecting said dies to said source to dry said sheet and restore the bond between plies thereof.

3. A method of molding an article of insulating material comprising the steps of soaking a sheet of laminated cellulosic insulating material in a softening liquid, placing said sheet on one o1' a pair of conductive dies while said dies are held in open position, causing plies of said sheet to yield and slide relatively to each other to thereby shape said sheet to conform to the said one die, closing said dies to press said sheet between said dies, and connecting said dies to a source of high frequency electric current to vaporize the liquid absorbed by said sheet. n

4. A method of molding an article of insulating material comprising the steps of soaking a sheet of laminated cellulosic material i-n a softening liquid", kneading said sheet to loosen the bond between the plies thereof, placing said sheet on one of a pair of conductive dies while said dies are held in open position, causing plies of said sheet to yield and slide relatively to each other to thereby shape said sheet to conform to the said one die, closing said dies to press said sheet between said dies to expel a portion of the liquid absorbed by said sheet and to perfect the molding of said sheet to the desired shape, and connecting said dies t0 a source of high frequency electric current to dry said sheet and restore the bond between plies thereof.

5. A method of molding an article of insulating material comprising the steps of soaking a `sheet of laminated cellulosic insulating material in a softening liquid, placing said sheet on one of a pair of conductive dies while said dies are held in open position, causing plies of said sheet to yield and slide relatively to each other to thereby shape said sheet to conform to the said one die, closing said dies to press said sheet between said dies under .a predetermined pressure to perfect the molding of said sheet to the desired shape and to expel a portion ofthe liquid absorbed by said sheet, and connecting said dies to a source of high frequency electric current to vaporize the remainder of the liquid absorbed by said sheet.

6. A method of molding an article of insulating material comprising the steps of soaking a, sheet @laminated eellulosic insulating materiel in .o softening liquid, placing, said sheet on one oi e pair of conductive dies while seid dies are held. in open position, causing plies of said sheet to yield and slide relatively to each other to thereby shape said sheet to conform to the said one die, closing said dies to a predetermined spacing to expel a portion of the water absorbed by said sheet and to perfect the molding of said sheet to predeterfmined dimensions, and connecting said dies to a source of high frequency electric current to Vaporize the remainder of the liquid absorbed by said sheet.

LOUIS C. AICHER.

REFERENCES CITED The following references are of record :1n 'the le of vthis patent:

UNITED STATES PATENTS 

